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Review

Modification of TiO2 Surface by Disilanylene Polymers and Application to Dye-Sensitized Solar Cells

by
Yohei Adachi
,
Daiki Tanaka
,
Yousuke Ooyama
and
Joji Ohshita
*
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan
*
Author to whom correspondence should be addressed.
Inorganics 2018, 6(1), 3; https://doi.org/10.3390/inorganics6010003
Submission received: 31 October 2017 / Revised: 21 December 2017 / Accepted: 22 December 2017 / Published: 26 December 2017
(This article belongs to the Special Issue Coordination Chemistry of Silicon)
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Abstract

:
The surface modification of inorganic materials with organic units is an important process in device preparation. For the modification of TiO2, organocarboxylic acids (RCO2H) are usually used. Carboxylic acids form ester linkages (RCO2Ti) with hydroxyl groups on the TiO2 surface to attach the organic groups on the surface. However, the esterification liberates water as a byproduct, which may contaminate the surface by affecting TiO2 electronic states. In addition, the ester linkages are usually unstable towards hydrolysis, which causes dye detachment and shortens device lifetime. In this review, we summarize our recent studies of the use of polymers composed of disilanylene and π-conjugated units as new modifiers of the TiO2 surface. The TiO2 electrodes modified by those polymers were applied to dye-sensitized solar cells.

Graphical Abstract

1. Introduction

Dye-sensitized solar cells (DSSCs) are of current interest because of the advantages they offer, including low fabrication cost and possible color tuning of the cells. The cells possess dye-attached TiO2 as photoactive electrodes [1]. This system involves electron injection from photoexcited dyes to the conduction band of TiO2 as the key step of the photocurrent generation. The resulting oxidized dyes are reduced by accepting an electron from the redox system, such as I2/I− in acetonitrile to recover the neutral state. Subsequently, electron-flow takes place from TiO2 to the redox system through electrodes generating the photo-current of the device. Conventionally, the modification of the TiO2 surface by organic dyes is performed by the formation of ester linkages between the Ti–OH bonds of the surface and the carboxylic acid groups of the dyes, as shown in Scheme 1 (1). However, the esterification produces water as a byproduct, which may contaminate the surface and thus change the electronic properties of TiO2. Furthermore, the ester linkages on the TiO2 surface are usually unstable towards hydrolysis and react with moisture to detach the dyes, shortening cell lifetime.
On the other hand, polymers having backbones composed of alternating organosilicon units and π-conjugated systems have been investigated as functional materials, such as carrier transporting and emissive materials [2,3,4]. Photoactive properties are also an important characteristic of Si–π polymers. In particular, those with Si–Si bonds are photoactive and UV irradiation of the polymer solutions leads to the cleavage of the Si–Si bonds. When the polymer films are irradiated in air, siloxane (Si–O–Si) and silanol (Si–OH) bonds arising from the reactions of the photoexcited Si–Si bonds with oxygen and moisture are formed. The formation of these relatively polar units increases the solubility of the polymers in alcohols, making it possible to utilize the polymer films as positive photoresists [2,5,6]. In fact, irradiation of the polymer films through a photomask followed by the development of the irradiated films by washing with alcohols provides sub–micron–order fine patterning. Utilizing the photoactivities, disilanylene–π alternating polymers are photochemically attached to the TiO2 surface through the formation of Si–O–Ti bonds, as presented in Scheme 1 (2). In addition, disilanobithiophene is also investigated as a binding unit to TiO2, and those resulting in polymer-attached TiO2 materials are applied to DSSCs. Hanaya and coworkers reported a similar modification of TiO2 electrodes by silanol and alkoxysilane dyes via the formation of Si–O–Ti linkages [7,8,9,10,11]. They demonstrated that the resulting electrodes show high performance as DSSC electrodes with high robustness towards hydrolysis in particular, as compared with electrodes with conventional ester linkages. For example, DSSCs based on a dye with a trimethoxysilyl anchor (1), shown in Chart 1, exhibited high performance with a power conversion efficiency (PCE) over 12%. This was higher than the DSSCs with a similar dye that had a carboxylic acid unit as the anchor (2) [9,11], clearly indicating the high potential of the Si–O–Ti bond as an efficient anchoring linkage. Dye 1-attached TiO2 showed higher stability towards hydrolysis and nearly no detach of the dye was observed after soaking for 2 h at 85 °C, while TiO2 with 2 underwent the liberation of approximately 70% of the dye under the same conditions. It was also demonstrated that an aminoazobenzene dye with a triethoxysilyl anchor (3) showed higher sensitizing ability than a similar one bearing a carboxylic acid unit (4) (Chart 1), because of an improved open-circuit voltage (Voc) arising from suppressed charge recombination [10].
In general, the anchors of the sensitizing dyes should have electron deficiency for the smooth electron injection from the photo-excited dye to the TiO2 conduction band. Silicon units are generally recognized as electron-rich units, because of the low electronegativity of silicon. However, it is also known that silicon substituents work as electron-accepting units when attached to π–electron systems. In this review, we summarize our recent studies of the use of the disilanylene polymers as new modifiers of the TiO2 surface. Applications of the modified TiO2 electrodes to DSSCs are also described.

2. Results and Discussion

2.1. Photochemical Attachment of Si–Si–π Polymers to TiO2 Surface

When a TiO2 electrode was irradiated (>400 nm) in a chloroform solution of poly[(disilanylene)quinquethienylene] (DS5T in Chart 2) with a Xe lamp bearing a cut filter, the colorless electrode turned yellowish brown [12]. In this process, light longer than 400 nm was used to avoid the activation of TiO2.
The photoreactions of compounds and polymers with Si–Si–π units have been studied in detail, and three types of reactions have been suggested as the major photodegradation pathways, as illustrated for poly(disilanylenephenylene) in Scheme 2 [2,5,6]: (1) 1,3-silyl shift from the disilanylene unit to the π–electron system forming silenes; (2) homolytic cleavage of the Si–Si bonds; and (3) direct reactions of the Si–Si bonds of the photoexcited molecules with alcohols. The alcoholysis of disilane units (route 3) proceeds dominantly over routes 1 and 2 when a large excess of alcohol is present in the reaction media [13,14]. The homolytic cleavage of Si–C bonds (route 4) is occasionally involved as a minor pathway, and routes 2 and 4 are preferred to route 1 in the polymeric systems [15,16]. However, the expansion of the π-conjugation usually suppresses the photoreactivities of the Si–Si–π compounds and polymers [17,18] and indeed, DS5T is basically not photoactive in an inert atmosphere [18]. Given these considerations, it seems most likely that the photochemical modification of the TiO2 surface with DS5T occurs via direct reactions of the photoexcited polymer with TiOH groups on the surface. The reactions of DS5T with water adsorbed to the TiO2 surface to form silanols and the subsequent condensation of the silanols with TiOH groups may also take place to form Si–O–Ti bonds.
A similar treatment of TiO2 with DS6T (Chart 2) also provided a polymer-attached TiO2. This modified TiO2 was examined as a photoelectrode of DSSCs. As presented in Table 1, the DSSCs showed photocurrent conversion, although the activities were not very high and the PCEs were approximately 0.1%. A colored TiO2 electrode was also obtained by dipping the electrode into a solution of the corresponding siloxane polymer DSO5T (Chart 2), presumably owing to the interaction between the polymer chain and the TiO2 surface, such as the coordination of the siloxane oxygen to the Lewis-acidic Ti site (Si2O–Ti) and hydrogen bonding to TiOH (Si2O–HOTi). However, the device with DSO5T-attached TiO2 showed much less efficient photocurrent generation (PCE = 0.05%), indicating that a chemically bound polymer on TiO2 is necessary to improve the activity. The attachment of poly[(ethoxysilanylene)quinquethiophene] (MS5T) on the TiO2 surface was also examined, as shown in Scheme 3 [19]. A DSSC with TiO2 modified by MS5T provided a PCE of 0.13% with Jsc (short-circuit current density) = 0.44 mA/cm2, Voc = 338 mV, and FF (fill factor) = 0.48.
The three-component hybridization of the polymer, TiO2, and carbon nanotube was also possible, as shown in Scheme 1 (3) [20]. Mixing DS5T with single-walled carbon nanotubes (SWNTs) by ball milling provided the hybrid material DS5T/SWNT, which was soluble in organic solvents. Irradiation of the TiO2 electrode in a solution of the hybrid gave a TiO2/SWNT/DS5T hybrid electrode. Application of the electrode to a DSSC led to improved performance with a PCE of 0.39%, which was 3.5 times higher than that based on the DS5T-modified TiO2 (Table 1). Improvement of the device performance was likely ascribed primarily to the enhanced carrier transporting properties by hybridization with SWNTs.
The rather low DSSC performance based on the DS5T- and DS6T-modified TiO2 was presumably due to the narrow absorption windows of the polymers, and thus donor-acceptor type π-conjugated systems were introduced to the Si–Si–π polymers in order to obtain more red-shifted absorption bands (Chart 2, Table 1) [21,22]. Photochemical treatment of TiO2 electrodes in the polymer solutions, similarly to that for DS5T and DS6T mentioned above, provided polymer-modified TiO2 that showed improved DSSC performance, as expected (Table 1). Among them, the best performance was obtained using the pyridine-containing polymer (DS4TPy). The pyridine unit would participate in a secondary coordinative interaction with the Lewis-acidic Ti site of the TiO2 surface to facilitate the electron injection from the photoexcited polymer to TiO2, as shown in Scheme 4. Enhanced electron injection through pyridine–Ti coordination has been reported [23]. Monosilane polymers with D–A type π-conjugated units (MS2TBt and MS2TBs in Chart 2) were also examined as DSSC dyes, which may attach to the TiO2 surface via coordination of the benzothiadiazole or selenadiazole units to the Lewis-acidic Ti sites [24]. Although the DSSCs based on these polymers showed photocurrent conversion, the performance was low with PCE < 0.1%, again indicating that chemical bonding to polymer is important to improve DSSC performance.
The thermal attachment of DS2E2TBt1 and DS4TPy to the TiO2 surface was also examined. However, DSSCs using the thermally modified TiO2 electrodes showed lower PCEs (0.17% and 0.23% for DS2E2TBt1 and DS4TPy, respectively) than those with the corresponding photochemically modified electrodes. This is due to the smaller amount of dye adsorbed to the surface, which was estimated to be approximately half of those of the photochemically modified electrodes.

2.2. Dithienosilole- and Disilanobithiophene-Containing π-Conjugated Polymers as Modifiers of the TiO2 Surface

In spite of our efforts to develop new and efficient sensitizing dyes for DSSCs based on Si–Si–π polymers, DSSC performance was rather low, with a maximal PCE of 0.40%. The absorption windows seemed to be still narrow even though the D–A type π-conjugated units were introduced to the polymers. This was presumably because the polymer π-conjugation was interrupted by disilanylene units, although there might be some interaction between the Si–Si σ-orbital and the π-electron systems, namely, σ–π conjugation [2]. In addition, the Si–Si bonds might be cleaved on photolysis to produce silyl radicals that compete with the TiO2 surface modification. The silyl radicals might add the π-electron systems to decompose the conjugated structures. We therefore prepared dithienosilole–pyridine fully conjugated polymers DTSPy and DTS2TPy (Chart 3), expecting that the polymers would interact with the TiO2 surface via pyridine–Ti coordination [25]. The polymers could be attached to TiO2 electrodes by dipping the electrodes into the polymer solutions without UV irradiation, and PCEs of 0.55% and 0.54% were obtained from the DSSCs based on DTSPy and DTS2TPy, respectively.
On the basis of these results, we designed and synthesized disilanobithiophene (DSBT)–pyridine and –pyrazine alternating polymers (Chart 3). We recently demonstrated that DSBT is an efficient donor unit of D–A π-polymers that are potentially useful as active materials of bulk hetero-junction polymer solar cells [26,27,28]. These DSBT–pyridine and –pyrazine polymers have fully conjugated systems in their backbones and show red-shifted absorption bands around 500 nm, as illustrated in Figure 1 [29]. They are able to attach to the TiO2 surface by both Si–O–Ti bonding and pyridine– or pyrazine–Ti coordination (Scheme 5). Interestingly, DSBT showed high reactivity arising from the ring strain and reacted with the TiO2 surface even in the dark. Indeed, homopolymer pDSBT that has no Lewis-base site could be attached to TiO2 by dipping a TiO2 electrode into the chloroform solution in the dark. As presented in Figure 2, the electrode thermally modified by pDSBT shows a darker color than that modified photochemically. This is most likely because the degradation of π-systems occurred to some extent under photochemical conditions, competing with the photo-derived modification of TiO2. In some cases, however, the thermally modified TiO2 electrode showed inferior performance as the photo-electrode of DSSCs to that modified under photochemical conditions, because smaller amounts of polymers could attach to TiO2 in the dark.
DSSCs using TiO2 electrodes modified by the DSBT–pyridine and –pyrazine polymers exhibited good performance with a maximal PCE of 0.89%, as presented in Table 2 and Figure 3, using a TiO2 electrode thermally modified with DSBTPz. Presumably, thermal modification in the dark led to the introduction of smaller amounts of polymers on the surface. However, in the photochemical modification, it is speculated that silyl and aryl radicals would be formed from the photo-induced homolysis of the Si–Si and Si–C bonds to some extent, as illustrated in Scheme 2, routes 2 and 4, as the minor photodegradation pathways for Si–Si–π polymers. The radicals add to the π-conjugated systems to suppress the conjugation, thus leading to the decreased efficiencies. In fact, DSSCs using photochemically modified TiO2 usually show IPCE (incident photon to current conversion efficiency) maxima at higher energies than DSSCs with thermally modified TiO2 [29]. As can be seen in Table 2, the performance changed depending on the conditions of attaching the dyes on TiO2. Some polymers showed higher performance when attached to TiO2 photochemically, but some others gave rise to better results under thermal conditions. Establishing a balance between the amount of polymer loaded and the degree of photodegradation seems important to further improve DSSC performance. This may be achieved by optimizing the polymer structure.

3. Conclusions

We have demonstrated that the reactions of Si–Si bonds with hydroxyl groups on the TiO2 surface provide an efficient route to modify the surface. These reactions proceeded cleanly without forming byproducts that might affect the properties of the TiO2. The Si–O–Ti bonds were known to be stable towards hydrolysis and seem to be useful for DSSCs with long lifetime. This process may be also applied to modify inorganic oxide surfaces other than TiO2, providing a hydrophobic surface with functional dye structures, thereby useful to control the surface and interface fine structures of organic optoelectronic devices such as organic thin film transistors and sensors. It has been also demonstrated that the attachment of azine-containing disilanylene polymers by both Si–O–Ti bonding and azine–Ti coordination improves the DSSC performance. This is likely ascribed to enhanced electron-injection through the azine–Ti coordination site. A similar function-separated dual site attachment of dyes on TiO2 electrodes by the simultaneous formation of an anchoring unit and an electron-injecting unit has been recently applied to DSSCs [30,31,32]. The present system with disilane and azine units as the anchoring and electron-injecting units, respectively, seems to provide a new molecular design for robust sensitizing dyes.

Acknowledgments

This work was partly supported by JSPS KAKENHI Grant Nos. JP26288094 and JP17H03105.

Conflicts of Interest

The authors declare no conflicts of interest.

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Inorganics 06 00003 sch001 550
Scheme 1. Modification of TiO2 surface with (1) organocarboxylic acid; (2) Si–Si–π polymer; and (3) s hybrid of Si–Si–π polymer and SWNT (single-walled carbon nanotube).
Scheme 1. Modification of TiO2 surface with (1) organocarboxylic acid; (2) Si–Si–π polymer; and (3) s hybrid of Si–Si–π polymer and SWNT (single-walled carbon nanotube).
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Chart 1. Structures of sensitizing dyes with a trialkoxysilyl or carboxylic acid anchor reported by Hanaya et al. and PCEs of the dye-sensitized solar cells (DSSCs) utilizing the dyes [9,10,11].
Chart 1. Structures of sensitizing dyes with a trialkoxysilyl or carboxylic acid anchor reported by Hanaya et al. and PCEs of the dye-sensitized solar cells (DSSCs) utilizing the dyes [9,10,11].
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Chart 2. Si–Si–π and Si–π alternating polymers for a DSSC.
Chart 2. Si–Si–π and Si–π alternating polymers for a DSSC.
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Scheme 2. Photodegradation of poly(disilanylenephenylene): (1) 1,3-silyl shift; (2) homolytic cleavage and (3) alcoholysis of Si–Si bond; and (4) homolytic cleavage of Si–C bond.
Scheme 2. Photodegradation of poly(disilanylenephenylene): (1) 1,3-silyl shift; (2) homolytic cleavage and (3) alcoholysis of Si–Si bond; and (4) homolytic cleavage of Si–C bond.
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Scheme 3. Reaction of MS5T with TiO2.
Scheme 3. Reaction of MS5T with TiO2.
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Scheme 4. Modification of TiO2 surface with DS4TPy.
Scheme 4. Modification of TiO2 surface with DS4TPy.
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Chart 3. DTS–pyridine and DSBT–pyridine polymers.
Chart 3. DTS–pyridine and DSBT–pyridine polymers.
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Scheme 5. Modification of TiO2 surface with DSBT–pyridine and –pyrazine polymers.
Scheme 5. Modification of TiO2 surface with DSBT–pyridine and –pyrazine polymers.
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Figure 1. Absorption spectra of DSBTPy and DSBTPz in o-dichlorobenzene. Reproduced from Reference [29]—Published by the Royal Society of Chemistry.
Figure 1. Absorption spectra of DSBTPy and DSBTPz in o-dichlorobenzene. Reproduced from Reference [29]—Published by the Royal Society of Chemistry.
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Figure 2. Photographs of (a) photochemically and (b) thermally modified TiO2 electrode by pDSBT.
Figure 2. Photographs of (a) photochemically and (b) thermally modified TiO2 electrode by pDSBT.
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Figure 3. IPCE spectra and J–V curves of DSSCs based on photochemically (solid line) and thermally (dashed line) modified TiO2 electrodes by DSBTPz. Reproduced from Reference [29]—Published by the Royal Society of Chemistry.
Figure 3. IPCE spectra and J–V curves of DSSCs based on photochemically (solid line) and thermally (dashed line) modified TiO2 electrodes by DSBTPz. Reproduced from Reference [29]—Published by the Royal Society of Chemistry.
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Table
Table 1. Polymer absorption maximum and DSSC performance.
Table 1. Polymer absorption maximum and DSSC performance.
Polymerλmax a/nmDSSC b Jsc/mA cm−2Voc/mVFFPCE/%
DS5T4360.762920.520.11
DS6T4180.862960.480.12
DSO5T c4260.572340.390.05
DS5T/SWNT-1.843400.620.39
DS2E2TBt14871.303080.610.25
DS2E2TBt24300.292280.460.03
DS2E2TBt35041.083920.600.26
DS4TBt5081.303240.630.28
DS4TBs5460.613240.570.11
DS4TPy4162.152960.630.40
MS2TBt c4510.423580.400.06
MS2TBs c4820.543360.490.09
a UV–Vis absorption maximum in solution; b FTO/TiO2–polymer/I2·I−/Pt; c dye-attached TiO2 electrode was prepared by dipping the electrode into the polymer solution without irradiation.
Table
Table 2. Polymer absorption maximum and DSSC performance.
Table 2. Polymer absorption maximum and DSSC performance.
Polymerλmax a/nmDSSC b Jsc/mA cm−2Voc/mVFFPCE/%
DTSPy c-2.174000.630.54
DTS2TPy c-2.033900.690.55
pDSBT4512.103080.610.39
pDSBT c5200.693560.670.16
DSBTPy4391.913440.630.41
DSBTPy c4841.673960.630.42
DSBT2Tpy4683.113800.630.74
DSBT2TPy c4751.343920.660.35
DSBTPz4681.583840.620.38
DSBTPz c4963.224240.650.89
DSBT2TPz14822.213960.640.56
DSBT2TPz1 c4892.284320.680.67
DSBT2TPz24902.703840.590.61
DSBT2TPz2 c5031.584200.660.44
a UV–Vis absorption maximum of polymer-attached TiO2 electrode; b FTO/TiO2–polymer/I2·I−/Pt; c Dye-attached TiO2 electrode was prepared by dipping the electrode into the polymer solution without irradiation.

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MDPI and ACS Style

Adachi, Y.; Tanaka, D.; Ooyama, Y.; Ohshita, J. Modification of TiO2 Surface by Disilanylene Polymers and Application to Dye-Sensitized Solar Cells. Inorganics 2018, 6, 3. https://doi.org/10.3390/inorganics6010003

AMA Style

Adachi Y, Tanaka D, Ooyama Y, Ohshita J. Modification of TiO2 Surface by Disilanylene Polymers and Application to Dye-Sensitized Solar Cells. Inorganics. 2018; 6(1):3. https://doi.org/10.3390/inorganics6010003

Chicago/Turabian Style

Adachi, Yohei, Daiki Tanaka, Yousuke Ooyama, and Joji Ohshita. 2018. "Modification of TiO2 Surface by Disilanylene Polymers and Application to Dye-Sensitized Solar Cells" Inorganics 6, no. 1: 3. https://doi.org/10.3390/inorganics6010003

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